1. Field of the Invention
The present invention relates to a Fresnel lens and a lighting apparatus provided with the Fresnel lens, and more specifically to a lighting apparatus using a Fresnel lens for an illumination spotlight used on stages and studios and a Fresnel lens used therein.
2. Description of the Related Art
A lighting apparatus is used in the stage lighting and the studio lighting, which is generally suspended from the ceiling. In a possibility of its fall, it is desired that lighting apparatus is light in weight from the viewpoint of safety. Generally, lighting apparatus is provided with a light source of high brightness. Thus, the lighting apparatus is heated to a high temperature due to a heat radiation from the light source during a lighting operation. Therefore, it is desirable that the lens used in the lighting apparatus has a relatively thin body for dissipating heat. The Fresnel lens has characteristics that realize a thin body and lightweight. Accordingly, in the lighting apparatus in this field, it has been common that Fresnel lens is used as lens for projecting light rays.
Generally, a convex lens is formed into a relatively thick body having a spherical surface or an elliptic surface to provide a sufficient lens power, and the weight thereof becomes heavy. In contrast, the Fresnel lens has a concentric arrangement of Fresnel lens segments corresponding to a concentric arrangement of segment surfaces of the convex lens. Therefore, the Fresnel lens has stepped lens surfaces having refracting surfaces equivalent to that of the convex lens. In a design of the Fresnel lens, a convex lens is divided into concentric convex lens segments, the thick portions of the respective convex lens segments are reduced and the segment surfaces of the convex lens segments are arranged as the Fresnel lens segments, and the Fresnel lens segments are concentrically arranged in the same manner. Since the Fresnel lens is not so thick as compared with the convex lens, it enables to realize a thin lens having the same lens power as the convex lens. Thus, the Fresnel lens can be made into a relatively thin lens without becoming thick, accordingly the lightweight property of the lens is attained, and heat does not collect in the lens, and a relatively good heat radiation characteristic can be obtained. From these characteristics, Fresnel lens is used as light distribution control lens that enable both spotlighting and floodlighting, in lighting apparatus for a stage lighting application or a studio lighting application where light source of high light intensity and large heat generation is used.
In the lighting apparatus used for the stage lighting and the studio lighting, it is preferable that an illuminating beam to be projected on a floor has a smooth and uniform illumination distribution without unevenness in brightness in its projected range. However, in lighting apparatus using conventional Fresnel lens, unevenness in illuminating intensity cannot be restrained, and sufficient stage effects cannot be attained.
The uneven brightness in the illuminating beam in the Fresnel lens arises from rise surfaces of the Fresnel lens segments. In a Fresnel lens having such a shape as cut out from a convex lens, steps are formed between Fresnel lens segments, and inevitably have the rise surfaces. The rise surfaces cause light rays that are reflected or refracted to the outside of radiation area, and the rise surfaces partially block the illuminating beam, and dark lines that are darker than the circumferences thereof occur in the illuminating beam, causing unevenness in brightness.
A Fresnel lens provided with a lens structure improving the unevenness of the illuminating beam has been disclosed in, for example, Jpn. Pat. Appln. KOKAI Publication No. 8-306216. This Fresnel lens is formed as a substantially meniscus lens whose bottom surface is not a flat but a substantially circular. In this Fresnel lens, the angle of the rise surface of each Fresnel lens segment with respect to the optical axis becomes larger depending on a radial distance from the center of the lens, and the rise surfaces are inclined in a radial direction. In such a Fresnel lens, the light rays incident on the rise surface are reduced, and uneven brightness in the illumination is decreased. Further, in Jpn. Pat. Appln. KOKAI Publication No. 8-306216, a Fresnel lens is proposed that the pitch of steps is made small so as to reduce unevenness in brightness.
Although designs to reduce uneven brightness in illuminating beam have been made in the conventional Fresnel lenses, it is pointed out that the conventional designs fail to reduce unevenness sufficiently. The reason is that if the light rays projected from Fresnel lens segments adjacent each other illuminate areas adjacent each other on a floor surface, the illumination areas are partly overlapped each other so that there is produced a relatively intense illumination part on the areas.
In respect to the illumination distribution on the floor surface, the illumination distribution is composed of substantially rectangular distribution components which are produced due to beam components from the lens segments and the adjacent rectangular distribution components are overlapped each other so that the total illumination distribution has sharp peaks corresponding to the overlapped distribution components. Owing to these sharp peaks, the conventional Fresnel lens has a characteristic of uneven brightness in the illuminating beam.
Further, even if a Fresnel lens is so designed as to have segment lenses which provide no overlapping between the illumination distribution components, the lens surfaces of the segment lenses may be slightly displaced owing to a production error or the like, as a result, overlapping of the illumination distribution components occurs, leading to illumination unevenness.
As described above, in lighting apparatuses using the conventional Fresnel lens, there has not been proposed any effective method to solve the uneven illumination, accordingly, the improved Fresnel lens is expected.